This invention relates to a magnetic disk drive, and more particularly to a magnetic disk drive in which the sector servo system is adopted.
Magnetic disk drives such that various information signals are recorded on a rigid magnetic disk accommodated within a casing and comprising a plurality of sectors forming concentric or helical tracks have been conventionally known. In the drives of this kind, the information signals described above are recorded or reproduced by a magnetic head moved in a radial direction of the magnetic disk rotated at a high speed.
Further, in the drives of this kind, a tracking servo control is carried out so that the magnetic head is made to track precisely with respect to an intended track of the concentric or helical tacks formed on the magnetic disk.
For such a tracking servo control, what is called the sector servo system is generally known.
Namely, in accordance with this sector servo system, servo signals are recorded in advance at head portions of respective sectors in a manner that they are shifted in a radial direction and in a circumferential direction (in a direction of time base) of the magnetic disk, respectively, the tracking servo control is performed so as to compensate an off-track quantity of the magnetic head obtained by comparative computation of reproduced output levels of these servo signals.
Such sector servo system is described in detail, e.g., in Japanese Patent Application No. 123103/1987.
Meanwhile, an error between an off-track quantity calculated and an actual off-track quantity may be produced in the conventional disk drives by the difference between reproduced output levels of the servo signals due to the difference in the magnetic head used, and/or by the difference in track width due to the expansion and contraction of the disk caused by changes in the ambient temperature.
Further, even if the same magnetic head is used, the reproduced output levels of the servo signals change depending upon the difference between the inner tracks and outer tracks of the disk. Also in this case, an error may be produced between an off-track quantity calculated and an actual off-track quantity.
In addition, since the reproduced output levels are different due to the difference in the magnetic head, the difference between the inner tracks and outer tracks of the disk, and the like, the detection sensitivity represented by the output of A/D conversion may vary for the reasons stated above in an apparatus in which an A/D conversion is applied to the reproduced output of the servo signal to calculate an off-track quantity.
They cause hindrance to realization of precise tracking servo control.